Our goal is the understanding of the mechanism of cellular immunity. It is known that lymphocyte mediators play a central role in these events. These substances are elaborated by antigen or mitogen activated lymph node cells. Their structure and their way of interaction with their target cell is not well understood. Our emphasis has been and will continue to be on mediators directed towards the macrophage. One of these, migration inhibitory factor (MIF), inhibits the migration of macrophages out of capillary tubes and is biochemically indistinguishable from macrophage activating factor (MAF), which enhances the defense capacity of macrophages. Our research will concentrate on the biochemical characterization of MIF which has been purified to homogeneity and its subsequent structural analysis. Furthermore, the biochemical and functional analysis of macrophage-associated esterases and esterase inhibitors will be undertaken. These substances appear to be involved in the control of the interaction of MIF with the macrophage. We also hope to apply successfully the results from these studies to the analysis of MAF.